The instant invention relates to thin layered silicon photovoltaic devices. Such devices typically have a substrate on which is grown a thin layer of high purity silicon. In the instant invention the substrate is formed of an alloy of silicon and boron.
Photovoltaic devices or solar cells convert solar radiation into useable electrical energy. When solar radiation impinges on a solar cell it is absorbed by an active region of semiconductor material to generate an electric current. This is known as the photovoltaic effect.
In the past, pure silicon has frequently been used for photovoltaic devices and to form such an active region.
However, the use of such high-purity silicon is much more expensive than metallurgical grade silicon. Achieving such high-purity silicon, or solar grade silicon, suitable for a solar cell, from lower-cost metallurgical grade silicon has been difficult. The number and extent of impurities in the metallurgical-grade silicon has inhibited its use.
Several publications have been written detailing the problems and methods of reducing the impurities in metallurgical grade silicon to achieve silicon of solar grade. They include "Silicon by Sodium Reduction of Silicon Tetrafluoride", (Sanjurjo et al., J. Electrochem Soc., 128, (1981) 179-184); "Progress On The Dow Corning Process For Solar-Grade Silicon", (Hunt et al., Proceedings, 2nd European Community Photovoltaic Solar Energy Conference, Berlin, Apr. 23-26, 1979, Reidel Publishing Co., (1979) 987-105); "Improved High-Purity Arc-Furnace Silicon for Solar Cells", (Amick et al, J. Electrochem. Soc., 132, (1985) 339-345); "Solar Cells From Metallurgical Silicon Zone Melted In Polycrystalline Silicon Tubes", (Jain et al., Solar Cells, 6 (1982) 357-363); and "Efficient Solar Cells from Metallurgical-Grade Silicon", (Proceedings of the 11th Conference (1979 International) on Solid State Devices, Tokyo, 1979, Japanese Journal of Applied Physics, Volume 19 (1980) 539-544). None of the above publications mention the deliberate addition of a substantial amount of boron to metallurgical-grade silicon to achieve a low-cost, highly conductive substrate suitable for epitaxial growth of good (high purity) thin silicon layers.
As an alternative to pure silicon solar cells, and to achieve significant cost reduction, a thin-film solar cell can be used. Such a cell includes a lower cost substrate upon which is deposited or grown a thin layer of semiconductor material. The benefits of such a thin-film cell are twofold; namely the cost savings associated with using less solar or photovoltaic grade silicon, and an improved performance due to the thinness of the silicon, since generated charge carriers have less travel distance to the cell's contacts. Such a thin-film solar cell is shown in U.S. Pat. No. 5,057,163 to Barnett et al. The substrate of the patent is a ceramic material comprising low purity silicon in the range of 20 to 90% by weight and up to 80% by weight of silicon carbide and/or glass. The addition of substantial amounts of boron to metallurgical grade silicon is not mentioned by the reference. Such a substrate as described in the patent requires, however, the addition of an active semiconductor layer, as the lattice matching required for good silicon growth is lacking.
Other low-cost substrates that have been used in the past include metals, synthetic organic resins, graphite, and other crystalline material. Such materials have proved to be acceptable through the addition of a low melting point crystalline film which is then recrystallized to form a large grain material on which the solar cell or semiconductor material can be grown. The use of substrates having a low melting point crystalline film deposited thereon is shown in U.S. Pat. No. 4,482,780 to Mitchell. Again, a problem with such substrates is that they do not provide a sufficient lattice match to have a silicon layer directly grown thereon. The addition of process steps to achieve a high-efficiency solar cell will correspond to the additional costs involved.
The subject invention avoids the problems of the prior art by providing a low cost substrate on which silicon or semiconductor material can be directly and effectively grown.